In Code Division Multiple Access (CDMA) systems, multiple, simultaneous users share radio resources through the use of unique, digital codes. CDMA is a form of spread-spectrum communications system, meaning that each channel shares the entire frequency spectrum. Interference between channels is maintained within acceptable limits through the use of orthogonal codes and strict power limits on transmitted signals.
CDMA has been implemented in cellular communications, examples of which are EIA/TIA standard IS-95, EIA/TIA standard IS-2000 (also known as cdma2000), and Universal Mobile Telecommunications Systems (UMTS).
The following discussion is based on the IS-95 and cdma2000 implementations of CDMA for cellular communications. It is to be recognized that this is merely exemplary of cellular implementations of CDMA systems, and that the invention is also applicable to cellular communications systems which are not CDMA based.
Cellular systems are comprised of a plurality of base stations interconnected by one or more switching systems. Each base station communicates with a number of mobile stations within a given geographical region known as a cell. The base station transmits data between its respective switching system and the mobile stations assigned to its cell. Communications from the base station to the mobile stations are over the forward or downlink channels. Conversely, communications from the a mobile station to its assigned base station are over the reverse or uplink channels.
Communications are initiated using shared uplink and downlink channels configured within the system. Typically, for base station-initiated communications, a Paging Channel is used. For mobile station-initiated communications, an Access Channel is used. Access channels are associated with specific Paging Channels. Under IS-95 and cdma2000, the forward CDMA channel may carry up to seven Paging Channels, and each Paging Channel may have up to thirty-two associated Access Channels.
The cdma2000 standard also supports Forward Common Control Channels in the downlink direction, and Enhanced Access Channels in the uplink direction which perform many of the same functions as the Paging Channels and the Access Channels when they are enabled. The discussion which follows uses the Paging Channel and Access Channel terminology.
The Access Channel is used by the mobile station to initiate communication with the base station and to respond to Paging Channel messages.
To initiate access, the mobile stations transmit on the Access Channel using a random access procedure. An access attempt consists of one or more access sub-attempts. In turn, each sub-attempt consists of a number of Access Sequences consisting of transmissions known as Access Probes. Each of the Access Probes consists of an Access Channel preamble and an Access Channel message capsule. The first Access Probe in an Access Sequence is transmitted at a specified initial power level relative to the nominal open-loop power level. Each subsequent Access Probe is transmitted at a power level that is a specified amount higher than the previous Access Probe.
After the transmission of each Access Probe, the mobile station waits for a specified period for an acknowledgement from the base station. If an acknowledgement is received, access is complete. If the acknowledgement is not received within the specified period of time, the mobile station continues with the rest of the Access Probes in sequence. All Access Probes within an Access Sequence are sent on the same Access Channel. If an Access Sequence does not succeed, the mobile station chooses a new Access Channel associated with the same Paging Channel by a PN randomization procedure, and sends the next Access Sequence of the access sub-attempt on the new Access Channel.
Most of the parameters of the random access procedure are provided by the base station to the mobile stations in the Access Parameters Message which is continuously transmitted on the Overhead Message Train.
Existing cellular systems conforming to the EIA/TIA cellular standards permit configuration of Access Overload Classes. Mobile stations are assigned to one (or more) of sixteen Overload Classes. The mobile station stores the class information in permanent memory. The base station controls which classes have access to the system by transmitting to the mobile stations the appropriate parameters for each Access Overload Class. The parameters are communicated to the mobile stations in the Access Parameters Message.
The EIA/TIA has defined the following Access Overload Classes: 0-9 for ordinary mobiles; 10 for test mobiles; 11 for emergency mobiles. The remaining Access Overload classes (12-15) are reserved.
The parameters relating to the Access Overload Classes as well as whether the Access Channel transmission is a registration, a message transmission or other transmission are used by the mobile stations to determine if and when they can initiate an Access Sequence as well as the delay between individual Access Sequences.
For incoming communications directed to a particular mobile station, the base station pages the mobile station using one of the Paging Channels configured within the system. The scheduling of pages is typically first-in, first-out.
The foregoing is specified and described in greater detail in the EIA/TIA standards IS-95 and IS-2000.
The IS-95 implementation of CDMA is what is known as a second generation communication system. Second generation communication systems are digital implementations which offer improved roaming capabilities over their analog predecessors. Other second generation communications systems include Global System for Mobile Communications (GSM), and Time Division Multiple Access (TDMA).
Third generation communications systems will be designed to provide global roaming, with high-speed data transmission. Examples of standards for third generation communication systems include Universal Mobile Telecommunications Systems (UMTS), and cdma2000. Third generation standards are still under development.
The deployment of third generation communications systems will result in an increase in the number of services that can be provided to cellular subscribers. The list of services includes, but is not limited to, voice, packet data, short message service, short data burst, and location management (registration). As traffic on communication networks increases, mobile stations using these services are forced to compete for resources including both the contention-based Access Channels and scheduled Paging Channels.
While existing third generation standards provide the limited ability to distinguish between classes of mobile stations which are accessing the system (through the Access Overload Classes), and between types of access attempts (registration versus message transmission versus other forms of transmissions), the standards do not provide a method for prioritization of the various services being used on the communications system. With all services being given equitable access to Access Channels and Paging Channels, cellular providers are not given the opportunity to distinguish between lower priority (e.g. less lucrative) and higher priority (e.g. more lucrative) services. In the result, the lower priority services acquire the system at the expense of the higher priority services.
In light of the foregoing, there is a need for a technique that permits cellular providers to prioritize access to system resources.